Silicate stages mineralogy in the later fractionation of the Insch intrusion, NE Scotland

Abstract

AbstractDespite poor exposures and lack of ‘stratigraphic’ control, there is consistent mineralogical and textural evidence that the Insch Upper Zone (UZ) represents a cumulate sequence developed from a single episode of progressive fractionation. This sequence has been subdivided on the basis of the phase layering, with cumulus plagioclase, pyroxenes and Fe-Ti oxides providing continuity from the Middle Zone (although the pyroxenes appear to be temporarily restricted to inter-cumulus status in the lower part of the UZ succession). The reappearance of cumulus olivine after its absence throughout the Middle Zone, marks the base of the UZ, and it is later joined by cumulus apatite, and then by alkali feldspar and zircon. Higher in the succession cumulus orthopyroxene, followed by olivine and Fe Ti oxide, eventually disappear. There is pronounced cryptic variation of the principal cumulus minerals (olivine Fo47−6, orthopyroxene En58−24, clinopyroxene Mg#63−6, plagio-clase An60−39 and the cumulus alkali feldspar is distinctly Ba-rich when it first appears. Comparison with the later fractionation stages of other layered intrusions shows broad similarities to the Bushveld and Skaergaard intrusions and closer similarities to the Fongen-Hyllingen body, which, like Insch, is a Caledonian synorogenic intrusion. The behaviour of the Insch pyroxenes, which show smoothly progressive Fe-enrichment despite their temporary absence as cumulus phases early in the UZ, together with the apparent range in cumulus plagioclase compositions in each sample, are taken to indicate that the cumulus mineral assemblages have been to some extent modified by re-equilibration with trapped inter-cumulus magma.